Fluids based on fluorocarbon compounds are widely used in vapor compression heat-transfer systems, especially air-conditioning, heat-pump, refrigeration or freezing devices. The common feature of these devices is that they are based on a thermodynamic cycle comprising vaporization of the fluid at low pressure (in which the fluid absorbs heat); compression of the vaporized fluid up to a high pressure; condensation of the vaporized fluid to liquid at high pressure (in which the fluid expels heat); and depressurization of the fluid to complete the cycle.
The choice of a heat-transfer fluid (which may be a pure compound or a mixture of compounds) is dictated firstly by the thermodynamic properties of the fluid, and secondly by additional constraints. Thus, a particularly important criterion is that of the environmental impact of the fluid under consideration. In particular, chlorinated compounds (chlorofluorocarbons and hydrochlorofluorocarbons) have the drawback of damaging the ozone layer. Non-chlorinated compounds are therefore now generally preferred, such as hydrofluorocarbons, fluoro ethers and fluoro olefins.
Another environmental constraint is that of the global warming potential (GWP). It is thus essential to develop heat-transfer compositions which have a GWP that is as low as possible and good energy performance qualities.
Moreover, to lubricate the moving parts of the compressor(s) of a vapor compression system, a lubricant oil must be added to the heat-transfer fluid. The oil may generally be mineral or synthetic.
The choice of lubricant oil is made as a function of the type of compressor, and so as not to react with the heat-transfer fluid itself or with the other compounds present in the system.
For certain heat-transfer systems (especially small ones), the lubricant oil is generally permitted to circulate throughout the circuit, the pipework being designed such that the oil can flow by gravity to the compressor. In other heat-transfer systems (especially large ones), an oil separator is provided immediately after the compressor, and also an oil level management device, which ensures return of the oil to the compressor(s). Even when an oil separator is present, the pipework of the system must still be designed such that the oil can return by gravity to the oil separator or to the compressor.
Document WO 2004/037 913 describes compositions based on fluoro olefins and especially based on tetrafluoropropene or pentafluoropropene. Example 2 reports the miscibility of 1,2,3,3,3-pentafluoropropene (HFO-1225ye) with various lubricant oils, and also that of 1,3,3,3-tetrafluoropropene (HFO-1234ze) with various lubricant oils. Example 3 reports the compatibility of HFO-1234ze and of 3,3,3-trifluoropropene (HFO-1243zf) with lubricant oils of the polyalkylene glycol type.
Document WO 2005/042 663 specifically concerns the miscibility of mixtures of fluoro olefins and of lubricant oils. The examples provided for these mixtures are essentially the same as those in document WO 2004/037 913.
Document WO 2006/094 303 describes a large number of heat-transfer compositions comprising fluoro olefins and additional compounds. Among the numerous compositions cited are mixtures based on 2,3,3,3-tetrafluoropropene (HFO-1234yf) and 1,1,1,2-tetrafluoroethane (HFC-134a). Moreover, the document generally teaches of combining the list of the numerous possible refrigerant mixtures with a list of lubricant oils.
When the heat-transfer compound(s) have poor miscibility with the lubricant oil, said oil has a tendency to be trapped in the evaporator and not return to the compressor, which does not enable correct functioning of the system.
In this regard, there is still a need to develop low-GWP heat-transfer compositions (which have good energy performance), in which the heat-transfer compounds show good miscibility with the lubricant oil.
In particular, HFO-1234yf is a heat-transfer compound that is particularly interesting especially due to its low GWP and its good energy performance. On the other hand, its miscibility with certain lubricant oils is imperfect and limits its application. It is thus desirable to improve the miscibility of compositions based on HFO-1234yf with the usual lubricant oils.